The present invention relates to a nipping device, more particularly to a nipping device which retains a printing sheet against the outer surface of a transfer unit, and separates it therefrom as well.
Generally a transfer drum comprising the principal component of a transfer unit is provided opposite a photoconductor drum in a full-color copying machine. A printing sheet is wound onto the circumferential surface of the transfer drum, wherein images developed on the photoconductor drum are transferred onto the printing sheet. The transfer drum is provided with a nipping device which is employed both to wind a printing sheet onto the transfer drum and to separate the printing sheet therefrom.
FIG. 13 shows a conventional nipping device, comprising lift members 71 having their tips inserted under a printing sheet 70, and a nipper 72 employed in conjunction with a stay of the transfer drum (not shown), to retain the printing sheet 70. A rotator shaft 73 is provided along the pivot axis of the lift members 71 and nipper 72, parallel to the axis of the transfer unit drum. Extending radially from the rotator shaft 73, are pins (not shown) which, as the rotator shaft 73 turns, come into lifting contact with the lift members 71 and nipper 72. One end 73a of the rotator shaft 73 forms a crank which rides on the outer surface of a cam 74 mounted on a rotator member within the transfer drum. Torsion springs 75 are disposed on the lift members 71 and nipper 72, whereby the lift members 71 and nipper 72 are impelled inward (toward the inside of the transfer drum). The lift members 71, nipper 72, rotator shaft 73 and torsion springs 75 are provided in the rotational element of the transfer unit. Separation claws 76 are fixed to a frame of the copying machine, adjacent to the transfer unit.
In the above-described conventional nipping device, when the rotation transfer drum is in position to nip a printing sheet, the crank portion 73a of the rotator shaft 73 riding on the cam 74 is driven to turn the rotator shaft 73 such that the nipper 72 is opened by the adjacent pin, separating from the circumferential surface of the transfer drum. At the point, a printing sheet 70 is fed in between the stay of the transfer drum and the nipper 72 by means of transport rollers, whereupon the nipper 72 is closed to nip the fed end of the printing sheet 70.
When the printing sheet 70 is transported by a transport system which is adjacent to the transfer drum after the image transfer operation is completed, the crank portion 73a of the rotating shaft 73 comes into contact with a cam different from the cam corresponding to the sheet nipping position, and the lift members 71 and nipper 72 are raised outward in the same manner as previously described. Consequently, the end of the printing sheet 70 is sufficiently separated from the circumferential surface of the transfer drum, to allow it to catch on the separation claws 76. As the transfer drum rotates further, the printing sheet 70 is transferred along the separation claws 76 to an adjoining transport system.
Since the conventional nipping device as described above employs torsion springs for impelling the lift members and nippers inward, and it is difficult to set and/or regulate the impelling forces of such springs, which must then of necessity be on the overly-powerful side, the drive system of the transfer drum is liable to receive excessive loads. Furthermore, since four sets of mechanisms as shown in FIG. 13 are normally provided in the transfer drum, a total of twelve torsion springs are thus required, and the tendency to excess load on the drive system is multipled.
Due to the fact that the load on the transfer drum drive system stemming from the impelling forces of the torsion springs is abruptly removed when the rotator shaft leaves its corresponding cam and the nippers and related elements are thereby closed, the change in load is of enough consequence to bring about undesirable effects upon the formation of an image therein.
It can also happen that the sides of a printing sheet of given size might not locate properly of the lift members, such that the printing sheet ends are not sufficiently lifted. This in turn can cause the printing sheet to bend up against the separation claws, resulting in a paper jam.